The achievement is bugged. You have to get 30 characters to level 30 to unlock it.
This is a guide on how you do this in an easy way.

The unlockable character Ronaldo have a passive skill that give +100% extra potion effect. This is the character we will level up several times!

Do the quest at Dead End Swamp where you collect Ectoplasm for Dexter. Before you complete the quest, equip 4 Ectoplasm. Dexter wont remove the Ectoplasm from your equipped items, so you get to keep them.

When you have 4 Ectoplasm equipped, you need 14 Greater XP Potions to get to level 28. If you don't have 4 Ectoplasm equipped you need 17 Greater XP Potions to get to level 28. This means that Ectoplasms will save you 300 gold every time you level up Ronaldo to level 30.

Go to the shop and buy the Soda Can and the Soda Bottle for a total of 120 gold.
Both the Soda Can and Soda Bottle will give you +50% extra potion effect.

Use your Greater XP Potions until you reach level 28.
Once you reach level 28, equip 4 Elite Harpy Beak instead. They give +25% potion effect each. Then use more potions.

To get to level 30 you need a total of 16 Greater XP Potions and one normal XP Potion.
When you get Ronaldo to level 30, delete Ronaldo, then repeat the process.

COSTS
4 x Elite Harpy Beaks = 1732g (If you don't already have them)
Soda Can + Soda Bottle = 120g
XP potions = 1620g (16 greater, 1 normal)
Adding Ronaldo = 250g (Can be lower)

Total cost is 3722g the first time, then 1870g each time you repeat the process.
Good Luck!